Method for using thin spacers and oxidation in gate oxides

ABSTRACT

A method for forming a lightly doped drain (LDD) field effect transistor uses very thin first sidewall spacers over the gate sidewalls, in which annealing/oxidation of the sidewall spacers results in (a) the rounding of corner portions of the gate structure sidewalls adjacent the gate oxide, and (b) a very low thermal consumption comprising a small portion of the total thermal budget. Secondary sidewall spacers of greater width are then formed to act as offsets in the introduction of N-type dopants into the substrate to form source and drain contact regions. The method may be varied to accommodate various design configurations and size scaling.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/840,855filed Apr. 24, 2001, U.S. Pat. No. 6,383,881, which is a continuation ofapplication Ser. No. 09/644,352, filed Aug. 23, 2000, now U.S. Pat. No.6,261,913, issued Aug. 23, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to integrated circuit devices. Moreparticularly, the instant invention pertains to a multiple implantlightly doped drain (“MILDD”) field effect transistor and method offorming the same.

2. State of the Art

Field Effect Transisters (“FET's”) are devices of choice in thefabrication of high density integrated circuits. In particular,manufacturers have scaled down FET circuit features to the sub-quartermicron level to achieve the high densities required for gigabit DynamicRandom Access Memories (DRAM's). The reduction of device dimensionsresults in a number of short-channel effects. One short-channel effectthat poses a primary obstacle to further reductions in scale is the“hot-carrier effect.”

In an N-channel FET, the hot-carrier effect occurs when the voltageapplied between the drain and source regions of the transistor increasesto a level at which the high lateral electric field in the transistorchannel induces impact ionization. During impact ionization, someelectrons are accelerated from the source region to the drain region andcollide with the silicon crystal lattice with sufficient kinetic energyto break chemical bonds in the lattice within the channel region of thetransistor.

As a result, free hole-electron carrier pairs are generated. The freeholes are attracted to the negatively-biased substrate, resulting in asubstantial increase in substrate current. At the same time, the freeelectrons are attracted to the positively-biased transistor gate.Although most of the free electrons completely traverse the gatedielectric layer, some become trapped within the dielectric layer. Whilethe channel-to-substrate current and channel-to-gate current results inincreased power dissipation, the injection and trapping of electrons inthe gate dielectric is far more serious because it causes the thresholdvoltage to increase and the current drivability to decrease.

During the operational life of the device, more and more electronsbecome trapped, resulting in a cumulative degradation of deviceperformance. Over time, the threshold voltage and current drivabilitycharacteristics may be degraded to levels that render the circuitinoperable.

Although an analogous process takes place in P-channel devices, it is ofless concern in practical applications. In a P-channel device, holesinstead of electrons are injected into and become trapped within thegate dielectric. The energy that must be imparted to a hole to cause itto be injected into the gate dielectric is substantially greater thanthe energy required for electron injection. The lower hole mobilityfurther reduces this effect. Far fewer carriers become trapped in aP-channel device, as compared to an N-channel device operating undersimilar conditions. As a result, the hot-carrier effect is potentiallynot as critical for P-channel devices. Accordingly, the discussion belowfocuses on N-channel devices, with the implicit assumption thatanalogous principles apply to P-channel devices.

As channel lengths, along with other device dimensions, decrease, thesupply voltage does not decrease proportionately. As a result, thelateral electric field in the channel is stronger for a given appliedvoltage. The stronger lateral field makes the hot-carrier effect morepronounced. In addition, the overall size reduction results in sourceand drain contact regions which are reduced in dimension. Accordingly,integrated circuit designers strive to reduce the hot-carrier effect insub-quarter micron scale devices, while simultaneously preventingdegradation of current drivability.

In the past decade, designers have taken a number of approaches tomitigate the hot-carrier effect, including: (1) increasing theresistance of gate oxide at the silicon—silicon dioxide (Si—SiO₂)interface through improved methods of dielectric processing, see Mathewset al., U.S. Pat. No. 5,393,683 (U.S. Class 437/42), issued Feb. 28,1995, entitled “METHOD OF MAKING SEMICONDUCTOR DEVICES HAVING TWO-LAYERGATE STRUCTURE”; (2) reducing the power supply voltage to reduce themagnitude of the lateral fields, an often difficult or impossibleapproach; (3) utilizing Lightly Doped Drain (“LDD”) FET's; and (4)utilizing other field-reducing device structures.

The most prevalent approach is to use standard LDD FET's. With thisapproach, the drain and source regions are doped with two differentimplants, one self-aligned to the gate electrode and the otherself-aligned to a sidewall spacer, which permits the implant to beoffset from the gate edge. Although this structure results in smallerlateral fields, it often leads to reduced drive currents. Disadvantagesof conventional LDD FET's include: (1) increased series resistancebetween the drain and source regions resulting from the existence of arelatively large lightly doped region adjacent to the channel; (2)spacer-induced degradation, resulting from the injection of carriersinto the spacer at the spacer-substrate interface, which results inincreased threshold voltage and reduced drive current. Designers havesuggested several modifications of the conventional LDD structure toimprove its electrical characteristics, with limited success. See, forexample, Ahmad et al., U.S. Pat. No. 5,405,791, issued Apr. 11, 1995,entitled “PROCESS FOR FABRICATING ULSI CMOS CIRCUITS USING A SINGLEPOLYSILICON GATE LAYER AND DISPOSABLE SPACERS”; Ahmad et al., U.S. Pat.No. 5,382,533, issued Jan. 17, 1995, entitled “METHOD OF MANUFACTURINGSMALL GEOMETRY MOS FIELD-EFFECT TRANSISTORS HAVING IMPROVED BARRIERLAYER TO HOT ELECTRON INJECTION”; and Gonzalez, U.S. Pat. No. 5,376,566,issued Dec. 27, 1994, entitled “N-CHANNEL FIELD EFFECT TRANSISTOR HAVINGAN OBLIQUE ARSENIC IMPLANT FOR LOWERED SERIES RESISTANCE.”

Three United States patents have been issued which relate to theformation of multiple nitride spacers to create a drain region havinggraduated controlled dopant concentrations. The result is a bettercontrolled electrical field strength over the drain contact region, agenerally reduced peak field strength, and reduced hot-carrier effects.These patents, all having the same inventorship and assignment as theinstant application, are U.S. Pat. No. 5,719,425, issued Feb. 17, 1998,entitled “MULTIPLE IMPLANT LIGHTLY DOPED (MILDD) FIELD EFFECTTRANSISTOR”; U.S. Pat. No. 5,866,460, issued Feb. 2, 1999, entitled“METHOD OF FORMING A MULTIPLE IMPLANT LIGHTLY DOPED DRAIN (MILDD) FIELDEFFECT TRANSISTOR”; and U.S. Pat. No. 5,998,274, issued Dec. 7, 1999,entitled “METHOD OF FORMING A MULTIPLE IMPLANT LIGHTLY DOPED DRAIN(MILDD) FIELD EFFECT TRANSISTOR.”

As disclosed in these three patents, anneal/oxidation of the spacers isperformed following the formation of each of a plurality of spacers.These spacers provide varying offset for making source and drain contactregions of graduated dopant concentration. In addition, a spacer may beused for opening the gate dielectric at the contact areas for ohmicconductors. These anneal/oxidation steps are conducted following doping,e.g., N-type doping to form the source and drain regions.

A portion of a prior art transistor formation on a substrate 10 isdepicted in drawing FIG. 1, having a gate structure 19 comprising apolysilicon layer 20, metal silicide layer 22 and cap 24 overlying agate dielectric 14. An oxidation step causes the edges 23 of thepolysilicon layer 20 adjacent the gate dielectric 14 and spacers 26 tooxidize and become rounded. As devices become increasingly smaller, theallowable thermal budget is necessarily decreased to prevent undesirableeffects from occurring. One such thermal effect is excessive diffusionof the dopants in the substrate 10, enlarging the contact regions 16 toshorten the channel 15, resulting in hot electron/carrier devicedegradation and other short-channel operation problems, as well asvariability in operational characteristics. The large spacers 26necessitate a high thermal budget to achieve the desired edge oxidationof the polysilicon layer 20, but excessive diffusion of contact regiondopants also occurs. Thus, in the manufacture of sub-quarter micronscale devices of further reduced scale, a method for preventingshort-channel effects such as hot electron/carrier degradation isrequired.

BRIEF SUMMARY OF THE INVENTION

In the instant invention, a method and apparatus are disclosed formanufacturing a field effect transistor (“FET”) in which short-channeleffects including a “hot-carrier” effect are mitigated.

The method comprises the formation of a very thin first spacer on thegate structure sidewalls, followed by an oxidation step. A second spacerof greater thickness is then formed on the thin first spacer to enableoffset N-type doping of the substrate, and opening of the gatedielectric for connection of ohmic contacts through the gate dielectricto the source and drain regions.

The present invention provides numerous advantages over the prior artLLD structures, including the following: (1) rounding of the corners ofthe sidewall-gate oxide interface is accomplished at minimal energyexpenditure, (2) a lower thermal budget is permitted, preventing undueexpansion of the source and/or drain regions by dopant diffusion, (3)short-channel effects including hot-carrier effects are mitigated,leading to better overall reliability and life, (4) current drivabilityand threshold voltage characteristics are not degraded, (5) noadditional masks are required, (6) a reduction of device overlapcapacitance is achieved, and (7) the method uses conventional processsteps.

DESCRIPTION OF THE DRAWINGS

The following drawings illustrate various embodiments of the invention,wherein:

FIG. 1 is a schematic cross-sectional side view of a portion of asemiconductor wafer undergoing a prior art process step using anexemplary gate sidewall spacer to fabricate a transistor;

FIG. 2 is a schematic cross-sectional view of a portion of asemiconductor wafer with gate structure thereon undergoing a processflow of the invention to fabricate a transistor;

FIG. 3 is a schematic cross-sectional view of a semiconductor waferundergoing a further step of a process flow of the invention tofabricate a transistor;

FIG. 4 is a schematic cross-sectional view of a semiconductor waferundergoing a further step of a process flow of the invention tofabricate a transistor;

FIG. 5 is a schematic cross-sectional view of a semiconductor waferundergoing an additional step of a process flow of the invention tofabricate a transistor; and

FIG. 6 is a schematic cross-sectional view of a semiconductor waferundergoing an additional step of a process flow of the invention tofabricate a transistor.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Referring to drawing FIGS. 2 through 6, depicted is a fabricationprocess for a Field Effect Transistor (FET) that corresponds to anembodiment of the present invention.

The following disclosure describes the formation of a single transistor;however, it is understood that in the usual case, a large number ofspaced-apart transistors are formed essentially simultaneously on awafer or wafer portion.

Referring to drawing FIG. 2, a step in the formation of a field effecttransistor (FET) 30 is depicted in a substrate 10 which comprises asemiconductor material, e.g., single crystal silicon, polysilicon(“poly-Si”) or amorphous silicon (“a-Si”), typically in wafer form.Substrate 10 has been doped with a P-type impurity, for example, boron.Field oxide 12 has been formed on substrate 10 to electrically isolateactive area 11 from other active areas on the wafer. The field oxide 12comprises silicon dioxide (SiO₂) and may be formed by any of severalwell-known methods, including the standard local oxidation of silicon(“LOCOS”) process.

A thin layer of gate dielectric 14 is formed superjacent substrate 10over active area 11. This invention is especially suitable for ultrathingate dielectric 14 having a typical thickness of about 80±20 angstroms,or less. Typical gate dielectric 14 will comprise SiO₂ that has beenformed using one of several well-known methods which result in very lowdefect density. A gate dielectric 14 comprising several sub-layers isalso possible. For example, a layer of silicon nitride (Si₃N₄) may besandwiched between two layers of SiO₂; the three layers are collectivelyreferred to as “ONO.” In either case, the oxide may be thermally grown,e.g., utilizing rapid thermal oxidation (“RTO”), or deposited, e.g.,utilizing chemical vapor deposition (“CVD”) or plasma-enhanced chemicalvapor deposition (“PECVD”).

Subsequent to formation of the gate dielectric 14, a gate structure 19is formed superjacent gate dielectric 14. Gate structure 19 hassidewalls 21. Gate structure 19 will typically comprise apolysilicon-metal-dielectric stack. For example, a layer of polysilicon20 is applied superjacent the gate dielectric 14 and has a typicalthickness of approximately 1800 Angstroms. Formed superjacent thepolysilicon layer 20 is a metal layer of, e.g., tungsten silicide(“WSi”) 22 which has a thickness typically of about 1800 Angstroms. Adielectric cap 24 of, e.g., Si₃N₄ is formed superjacent the tungstensilicide 22 and may have a thickness of about 2400 Angstroms forexample.

To further reduce the resistance of the metal layer, the single layer ofWSi 22 may optionally be replaced by a first sublayer of WSi, not shown,superjacent the polysilicon layer 20, and a second sublayer of tungsten(“W”), not shown, superjacent the WSi sublayer.

Each of the three layers 20, 22 and 24 is formed by firstblanket-depositing the appropriate material superjacent the wafer, usinga photolithographic mask to define the location of each gate structure19, and then etching the material from all locations on the wafer otherthan the location of each gate structure 19. Where the sidewalls 21 arevertical, the same mask may be used for each of the three layers 20, 22,24 because each layer is aligned with the other two layers.

The metal silicide layer 22 provides the gate structure 19 with highconductivity. One of several alternatives to WSi is titanium silicide(“TiSi₂”). Optionally, the single layer of WSi may be replaced with afirst sublayer of WSi, not shown and a second sublayer of tungsten(“W”), not shown.

The polysilicon layer 20 acts as a transition layer between twootherwise chemically incompatible layers, namely the gate dielectriclayer 14 and the metal silicide layer 22, and also results in improvedelectrical characteristics of the device.

The dielectric cap 24 protects the gate structure 19 from subsequentprocessing steps. A nitride cap is preferred over a SiO₂ cap, becausethe latter material is vulnerable to subsequent contact etches of SiO₂.Many other gate structure configurations are possible. Variations inmethods for fabricating the gate structure 19 are known in the art, andwill not be specifically described herein.

The principal purpose of the gate dielectric 14 is to prevent currentflow between the gate structure 19 and the substrate 10. However, thegate dielectric 14 is left in place between the gate structure 19 andthe field oxide 12 to protect the surface 13 of the substrate 10 fromsubsequent process steps, i.e., in order to protect the surface'sintegrity. Additional oxide layers, not shown, may also be deposited toincrease the thickness of gate dielectric 14 in areas where processsteps, e.g., the introduction of dopants, are to be performed by, e.g.,introducing the dopants into the surface 13 of substrate 10. Theseadditional oxide layers can provide improved surface protection duringion implantation process steps, especially when the gate dielectric 14is relatively thin.

After the gate structure 19 is formed, an N-type dopant is introducedinto substrate 10 at those locations on the wafer where the field oxide12 and the gate structures 19 do not act as barriers to the N-typedopant. The gate structure 19 acts as an “offset” or partial mask tolimit dopant penetration in areas of the substrate 10 beneath the gatestructure. In this manner, N-type contact subregions 16A, 16B are formedin the P-type substrate 10, where subregion 16A is referred to herein asa “source contact region,” and subregion 16B is referred to as a “draincontact region.” The term “contact” refers to the fact that a primarypurpose of these regions 16A, 16B is ohmic contact of each region to anexternal interconnect, not shown. The P-type region 15 between thesource contact region 16A and drain contact region 16B is hereinafterreferred to as a “channel region.” The terms “source” and “drain” referto the flow of electrons when the transistor is “turned ON” andoperating in its normal mode, in which case a positive bias is appliedto the drain contact region 16B, causing electrons to flow from thesource contact region 16A to the drain contact region 16B via thechannel region 15. In the example presented herein, the doped draincontact region 16B is termed a “lightly doped drain.”

It can be appreciated that as the wafer is subjected to subsequentthermal processing steps, the N-type dopant in the contact subregions16A, 16B continues to diffuse away from its points of introduction intothe substrate 10. It is highly desirable that a manufacturer be able tocontrol with some precision the depth and lateral profile, i.e., outerboundaries of the contact regions 16A, 16B because the expansion ofthese regions into the P-type substrate 10 can affect the ultimatedesign parameters of the transistor 30. Generally, shallow contactregions 16A, 16B are preferred because they have to be scaled inaccordance with lateral dimensions to maintain adequate deviceperformance. Therefore, the use of N-type dopants having low diffusionrates is preferred. Suitable N-type dopants include phosphorus (P),arsenic (As), and antimony (Sb). As and Sb are preferred over phosphorusbecause they have especially low diffusion rates. As is generallypreferred over Sb as a dopant because the properties of As are morefully understood, although Sb has a lower diffusion rate than does As attemperatures lower than about 1000 C, and Sb also exhibits a lowerautodoping effect.

The N-type dopant is preferably introduced into the substrate 10 by ionimplantation. The energy of the implant could range approximately from20 keV to 130 keV. The preferred energy would range approximately from30 keV to 100 keV, depending upon the desired junction depth. The dosagewill depend upon the dopant concentration that is desired at the edge ofthe gate structure 19 for the contact regions being created, which inturn will depend upon the device performance and characteristics desiredor required for the particular application.

Alternatively, the N-type dopant could be introduced into the substrate10 by other known means, including plasma doping, in which ions in aplasma are driven into a substrate under the influence of an electricalfield, and other diffusion methods.

In a conventional process flow, only one source region 16A and one drainregion 16B are formed for each transistor. In a more recent development,a series of source regions and drain regions are sequentially formed.The present invention will be illustrated in terms of a single sourceregion 16A and a single drain region 16B, but may be applied to anynumber of such regions.

In accordance with the present invention, drawing FIG. 3 shows a furtherstep in the production of a FET 30. A first or primary sidewall spacer26 is formed on one or both gate sidewalls 21. Primary sidewall spacer26 is preferably formed of a nitride such as Si₃N₄ because of itsresistance to etching during subsequent oxide etch steps, e.g., contactetch steps. The primary spacer 26 may be formed by blanket depositing athin conformal layer of silicon nitride over the entire wafer and thenanisotropically dry etching the entire wafer in the vertical direction.In this manner, all of the nitride is removed except a very thin spacer26 adjacent the steep vertical sidewalls 21. The primary (first)sidewall spacer(s) 26 will have a very limited thickness of about 100Angstroms, and typically in the range of between about 50 and 150Angstroms. As the size scale of field effect transistors 30 is furtherreduced, the thickness of the primary spacer(s) 26 may be reduced evenfurther.

As depicted in drawing FIG. 4, formation of the primary sidewall spacers26 by dry etching is followed by a controlled anneal/oxidationatmosphere 25 to anneal the first spacers 26 and to oxidize and roundthe corners of the gate oxide-sidewall interfaces, all with a minimumthermal usage. The oxidizing atmosphere 25 results in oxidation androunding of the edges 23 at the interface between gate dielectric 14,polysilicon layer 20 and primary sidewall spacer 26. Because the primarysidewall spacers 26 have such a low mass, the thermal budget of theoxidation step is much lower than would otherwise be the case. The lowthermal use minimizes diffusion of N-type dopant from the N-type dopedcontact regions 16A, 16B into the substrate 10, and the appearance ofchannel shortening is substantially avoided. Excessive diffusion is ofever-increasing importance as the size of device features is furtherreduced. A low thermal budget is required for forming the shallowjunctions in FET's 30 of very small dimensions. The operatingcharacteristics of the FET 30 are affected adversely by enlarged contactregions 16A and 16B, including electron trapping and poor leakagecharacteristics, degraded drive current and non-optimal thresholdvoltage. Such factors may lead to failure of the device.

As shown in drawing FIG. 5, following the oxidation step, secondarysidewall spacers 28 are formed over the primary sidewall spacers 26.These secondary sidewall spacers 28 are of greater thickness than thevery thin first spacers 26, and in this example have a thickness whichprovides for light doping of subregions 17A, 17B.

The thickness of secondary sidewall spacers 28 may be conformed toprovide the desired contact spacing. Contact spacing is, of course, afunction of the scale of the FET 30. Typically, the thickness of thesecondary sidewall spacer 28 is about 2.0 to 20 times the thickness ofthe primary sidewall spacer 26, and for example, may be about in therange of 400 to 1000 Angstroms. The secondary sidewall spacers 28 arepreferably formed of the same dielectric material as the primarysidewall spacers 26, including silicon oxide and silicon nitride. Thelatter is preferred because of its resistance to oxide etching (contactetch). The primary purpose of the secondary sidewall spacers 28 is toprovide an offset for the subsequent introduction of N-type dopants 35into the substrate 10. In addition, the secondary sidewall spacers 28may have a thickness which locates the ohmic conductors in the desiredpositions. The dopants 35 may be introduced in one of the same mannersas previously noted, including ion implantation, plasma doping, andother doping methods. Where primary or first N-type contact regions 16A,16B have been previously formed, secondary contact regions 17A, 17B areformed by offset of the secondary sidewall spacers 28. Contact regions17A, 17B are typically formed to co-occupy portions of contact regions16A, 16B. Co-occupied portions will have a higher concentration ofdopant, having been subjected to two (or more) doping steps rather thanone.

As already indicated, the secondary sidewall spacers 28 may also be usedto open the contact regions for connection to device leads. The spacers26, 28 may be left in place during subsequent manufacturing steps, andbecome part of the device.

In the illustrated embodiment, shown in drawing FIG. 6, the secondarysidewall spacers 28 may be further etched, resulting in third spacers 29of thickness intermediate to the first and second spacers. Light dopingis then performed, resulting in a series of contact subregions 16A, 16Bhaving low dopant concentrations, contact subregions 17A, 17B havingintermediate dopant concentrations and contact subregions 18A, 18B withrelatively high dopant concentrations. The latter subregions 18A, 18Bhave of course been doped three times, subregions 17A, 17B doped twice,and subregions 16A, 16B doped once.

If desired, spacers of further thicknesses may be used to formadditional contact subregions of different dopant concentrations.

In a further embodiment of a method of the invention, each spacer in aseries of spacers has a greater thickness than the previous spacer. Theconsecutive order of figures illustrating this embodiment is drawingFIGS. 2, 3, 4, 6, and 5. Further spacers, not illustrated, of evengreater thickness, may be used for additional doping of the source anddrain regions. The final spacer may be left in the transistor 30 ifdesired.

Although the present preferred embodiment of the invention has beendescribed in the context of conventional silicon technology, theinvention may be used in conjunction with silicon-on insulator (“SOI”)wafer technology. In a SOI wafer, the contact subregions could extendall the way through the silicon substrate to the dielectric layer.

Each United States patent referenced herein is hereby incorporated byreference thereto as if set forth in its entirety. Although we haveillustrated and described a present preferred embodiment of theinvention and variations thereon, the invention is not limited theretobut may be embodied otherwise within the scope of the following claims.

What is claimed is:
 1. A manufacturing method for a transistor on asubstrate, comprising: providing a substrate having a dielectric layerthereon; forming a gate structure on said dielectric layer having a gateoxide layer formed on said dielectric layer and a metal suicide layerformed on said gate oxide layer, said gate structure having a firstsidewall and a second sidewall, said first sidewall and said secondsidewall defining therebetween within said substrate a first contactregion, a channel region and a second contact region; and forming first,second, and third subregions within said second contact region, eachsubregion having a dopant concentration that differs from that of theother two subregions, said forming of said first, second, and thirdsubregions comprising: depositing a conformal layer of dielectricmaterial over said substrate; anisotropically etching said conformallayer of dielectric material, forming a layer of dielectric material onsaid first sidewall and said second sidewall; subjecting said layer ofdielectric material on said first sidewall and said second sidewall toan annealing/oxidation process; forming a single layer sidewall spaceroverlying said first sidewall and said second sidewall; introducing afirst dopant into said substrate to form said first subregion; forminganother single layer sidewall spacer overlying said single layersidewall spacer; introducing a second dopant into said substrate to formsaid second subregion; substantially removing said another single layersidewall spacer; and introducing a third dopant into said substrate toform said third subregion.
 2. The method of claim 1, wherein said singlelayer sidewall spacer comprises a layer having a thickness in a range ofbetween about 50 and 150 Angstroms.
 3. The method of claim 1, whereinsaid another single layer sidewall spacer comprises a layer of materialhaving a thickness in a range of about 2 to 20 times a thickness of saidsingle layer sidewall spacer.
 4. The method of claim 1, wherein saidanother single layer sidewall spacer comprises a layer of materialhaving a thickness of about 550 Angstroms.
 5. The method of claim 1,wherein said another single layer sidewall spacer comprises a materialof one of silicon nitride and silicon dioxide.